Alkaline batteries, in particular, discharge starting type alkaline batteries or alkaline primary batteries have an inside-out type structure in which a cylindrical positive electrode material mixture pellet is disposed in a positive electrode case serving as a positive electrode terminal such that the pellet is in close contact with the positive electrode case, and a gelled zinc negative electrode is disposed in a hollow portion of the pellet with a separator interposed therebetween. With the recent widespread use of digital devices, the load power of the devices for which these batteries are used has been gradually increasing, and there has been a demand for batteries having excellent heavy load discharge characteristics. In response to such a demand, an alkaline battery whose heavy load discharge characteristics have been improved by mixing nickel oxyhydroxide into a positive electrode material mixture has been proposed, and this has been recently put into practical use (Japanese Unexamined Patent Publication No. Sho 57-72266).
On the other hand, in the field of alkaline storage batteries (secondary batteries), nickel oxyhydroxide that is obtained by oxidizing spherical or oval nickel hydroxide with an oxidizing agent such as a sodium hypochlorite aqueous solution is generally used. As the source material nickel hydroxide, nickel hydroxide having a high bulk density (tap density) and a β-type crystal structure are used. Nickel oxyhydroxide that is obtained by treating this with an oxidizing agent mainly has a β-type crystal structure, and can easily be filled at a high density inside a battery. The nickel oxyhydroxide having a β-type crystal structure has a nickel valence that is substantially 3, and the electrochemical energy generated when this changes to a valence near 2 is utilized as the discharge capacity of a battery.
For the purpose of increasing, for example, the utilization of the positive electrode and the heavy load discharge characteristics, there has also been proposed a technique that uses a solid solution nickel hydroxide in which cobalt, zinc or the like is dissolved, as the starting material (Japanese Examined Patent Publication No. Hei 7-77129).
Examples of the challenges that alkaline primary batteries containing nickel oxyhydroxide face are as follows:
(a) Improvement for the self decomposition (the decrease in the capacity and the increase in the internal pressure of the batteries) of nickel oxyhydroxide that occurs during storage of the batteries under a high temperature atmosphere.
(b) Improvement for the low discharge capacity (discharge duration) due to the small capacity per unit weight (mAh/g) of nickel oxyhydroxide.
In order to solve the above-described challenges, the following proposals have been made for the positive electrode material mixture of alkaline primary batteries.
First, from the viewpoint of improving the storage characteristics, it has been proposed to contain, in nickel oxyhydroxide, at least one oxide selected from the group consisting of a zinc oxide, a calcium oxide, an yttrium oxide and titanium dioxide (Japanese Unexamined Patent Publication No. 2001-15106).
Further, in the alkaline storage battery applications, it has been proposed to use a solid solution nickel hydroxide having a β-type crystal structure and including a transition metal such as manganese dissolved in its particles, as the starting material (International Publication No. WO 97/19479 and the specification of Japanese Patent No. 3239076). Here, nickel oxyhydroxide having a γ-type crystal structure and an average valence of nickel near 3.5 was intentionally formed during the charge reaction, thereby increasing the capacity significantly.
As a technique similar to this, for example, Japanese Unexamined Patent Publication No. 2001-322817 has proposed the use of particles of an α-type solid solution nickel hydroxide that was produced by coprecipitating ions of a transition metal such as manganese or iron that are in trivalent state with divalent nickel ions, as the starting material. Here, nickel oxyhydroxide having a γ-type crystal structure is formed during charge, thus increasing the capacity.
Further, it has been proposed to improve the discharge characteristics by coating the surface of particles of nickel oxyhydroxide having a γ-type crystal structure with a cobalt oxide having a high electrical conductivity (Japanese Unexamined Patent Publication Nos. Hei 10-334913 and Hei 11-260364).
However, any attempt to increase the capacity by using nickel oxyhydroxide having a γ-type crystal structure for the positive electrode has not yet been put to practical use for alkaline storage batteries. The reason lies in that a γ-type crystal excessively absorbs an electrolyte and thus expands in volume, so that the electrolyte distribution in the batteries greatly changes during the first several tens of charge/discharge cycles. When the electrolyte is localized on the positive electrode side and thus the electrolyte becomes insufficient in the separator, the internal resistance in the battery significantly increases.
On the other hand, the present inventors attempted to use, for primary batteries, nickel oxyhydroxide having a γ-type crystal structure, which has been investigated for alkaline storage batteries, and investigated the problems that could occur in such a case.
First, in the case of increasing the energy density of an alkaline primary battery containing nickel oxyhydroxide, one possible approach is to set strong chemical oxidation conditions for a source material nickel hydroxide having a β-type crystal structure, thereby increasing the nickel valence of the resulting nickel oxyhydroxide having a β-type crystal structure. Such an approach, however, can only provide nickel oxyhydroxide having a β-type crystal structure and in which the upper limit of the nickel valence is less than 3.00 to 3.05.
Then, it was found that the heavy load discharge characteristics tended to decrease more easily in the case of alkaline batteries as primary batteries that used nickel oxyhydroxide having a γ-type crystal structure, than in the case of alkaline batteries that used nickel oxyhydroxide having a β-type crystal structure, for the reasons shown in (a) to (c) below.
(a) The redox potential (equilibrium potential) of nickel oxyhydroxide having a γ-type crystal structure is lower than that of nickel oxyhydroxide having a β-type crystal structure.
(b) Nickel oxyhydroxide having a γ-type crystal structure undergoes a large volume change (change in the crystal structure) that is caused during discharge.
(c) The electron conductivity of nickel oxyhydroxide having a γ-type crystal structure and including manganese dissolved in its particles greatly decreases with discharge.
For primary batteries such as nickel-manganese batteries, nickel oxyhydroxide is added to the positive electrode material mixture, in order to compensate for the disadvantage of a low utilization of manganese dioxide during heavy load discharge. However, the above-described finding means that a γ-type crystal structure may significantly impair the advantage that nickel oxyhydroxide improves the heavy load discharge characteristics of alkaline batteries.